Only able to park one call

Using FOP2 2.31.02 with FreePBX 12 we are able to transfer a single call quite normally into the parking lot (default parking lot is on extension 70 with 9 parking slots from 71 to 79). However if we try to move another call into the parking lot this appears to kick the first call out of parking and this first calls then hangs up leaving the second call as the only call in the parking lot. Using just a normal transfer (*270) to transfer calls into parking this does not happen and we can park more than one call.